Atrioventricular transmission is a major electrophysiological event occurring in every cardiac cycle. In large mammals, because of the complexity of the deep AV junctional tissues and the inavailability of an adequate model for in vitro study, our knowledge on the cellular mechanism of AV conduction is limited. The study proposed is to develop an isolated canine atrial muscle-paranodal fibers-AV node-His bundle preparation by dissection to fill this need. The AV junctional tissues of this preparation are excited by antegradely propagated impulses elicited at the artrial tissue. Microelectrode techniques together with His electrogram recordings are used to monitor their electrical activities. The following studies will be performed. 1.) To demonstrate the electrical and physical (use histology technique) continuity of tissues of the preparation. The finding is expected to provide a functional and morphological evidence supporting a new concept of presence of a prominent AV conduction pathway running parallel and adjacent to the spetal cusp of the tricuspid valve. 2.) The suppressive effects of acetylcholine on impulse conductivity and the critical site responsible for maximal slowing of impulse conduction or conduction block. This finding provides insights as to what could happen in the AV conduction system at the cellular level under vagal influence. 3.) Facilitative effect of epinephrine on velocity and rate of impulse conduction. This finding provides insights as to what could happen at the cellular level in the AV conduction system under sympathetic influence such as during exercise. This isolated preparation opens the gate for future direct study on canine AV junctional tissues, not only on normal and abnormal electrophysiology but also on morphology, biochemistry and pharmacology. Following the completion of this proposed study, my next research objective is to determine if there is an alternate atrial input to the compact AV node or the ionic mechanisms responsible for the action potentials of each of the AV junctional tissues.